Boost converter type voltage regulators are well known in the art. Such regulators provide an output voltage greater than or equal to the input voltage and in one form comprise an input filter capacitor, a shunt switch, an energy storage element such as an inductor in series between the switch and the regulator input, an output capacitor and control means to operate the switch to achieve a desired regulator output voltage. Typically, the switch is operated at a high rate with each closing operation serving to store energy in the inductor and each opening operation effecting a transfer of the stored energy to a load connected to the regulator output. Boost converter type regulators are normally applied to low impedance voltage sources wherein the source voltage remains substantially constant over a range of source current. As is well known, in such applications the duty cycle of the switch, i.e. the portion of time within each switching cycle that the switch is closed, is varied in direct relation to the desired change in the regulator output voltage. That is, in response to an undesirably low regulator output voltage, e.g. due to an additional load demand or a decrease in source voltage, the duty cycle is initially increased in order to increase the output voltage. Correspondingly, the response to an excessive regulator output voltage is a decrease in the duty cycle which is effective to decrease the output voltage.
Shunt type regulators are also well known in the art, differing in a number of respects from the boost converter type regulator described above. Such regulators comprise a shunt switch, an output filter section and control means to operate the switch in a manner effective to achieve a desired regulator output voltage. The switch is operated in the same frequency range as the boost converter type regulator. However, each closing operation of the switch effectively short circuits the source. Thus, such regulators are normally applied only to high impedance sources which provide only a small variation in current over a wide range of source voltage. Applied to such a high impedance source, the duty cycle of the shunt switch is adjusted to pass to the load, while the switch is open, only so much source current as is required to maintain a desired regulator output voltage. The balance of the current is shunted away on a time average basis while the switch is closed. Thus, an increase in the duty cycle has the effect of reducing the regulator output voltage since less current is passed to the load. As a result, the shunt regulator is operated with a reverse control law. That is, in order to increase the output voltage, the duty cycle is decreased to pass more current to the load. Correspondingly, the duty cycle is increased to decrease the regulator output voltage.
A major disadvantage to the use of a shunt regulator is the electromagnetic interference (EMI) its operation generates. Since each closing of the shunt switch effectively short circuits the source, the input voltage of the regulator assumes the appearance of a square wave. Such a square wave voltage generates significant EMI. The ripple present in the corresponding current flow from the source also contributes to the adverse EMI effects but to a lesser degree since the magnitude of the current variation is limited by the high source impedance. In a situation where the high impedance source is a multiple winding alternator, the generated square wave voltage may be coupled by transformer action within the alternator to other loads supplied by the alternator.
An additional disadvantage of the shunt regulator as compared to the boost converter type regulator is that the former conducts a higher magnitude of current upon each switch closure. As a result, correspondingly higher resistive losses may be experienced in the shunt regulator.
Linear series and shunt dissipative type voltage regulators, well known in the art, do not by the nature of their operation generate any EMI. Such regulators may be applied to both high and low impedance sources. However, linear dissipative regulators dissipate real power in order to provide a regulated output voltage and are therefore inefficient in operation.
A boost converter type regulator, by the nature of its operation, does not generate square wave voltages at its input and the corresponding EMI. However, such regulators, insofar as is known, have not been applied to a high impedance source and its corresponding wide open circuit voltage range. Operated in the above-described manner, the duty cycle of the shunt switch in a boost converter regulator would be increased in order to increase the regulator output voltage. However, an increase in duty cycle would effectively reduce the source voltage, due to the high source impedance, and thereby cause less power to be transferred to the load and the regulator output voltage to decrease. This is contrary to the desired result, since more rather than less power is needed in order to increase the regulator output voltage. Thus, a degenerative situation would follow wherein an increase in the duty cycle would result in a decrease in regulator output voltage, the control response to which would be a further increase in the switch duty cycle.